Polyester production from aromatic dicarboxylic acids or their esters such as dimethyl terephthalate (DMT), and glycols is known. This has been accomplished by stage-wise melt polymerization of the dihydroxy ester of the aromatic dicarboxylic acid, or low molecular weight oligomers thereof, under successively higher vacuum conditions. In order for the polymerization to continue to the degree needed for most commercial applications, the condensation by-products, especially ethylene glycol, must be removed from the reaction system at vacuums as high as 1-3 mm Hg. Such processes require costly high vacuum equipment, multistage steam jets to create the vacuum, and N.sub.2 purged seals and flanges to minimize leakage of air into the system. Condensate from the steam jets and organic by-products from the system end up as a waste water stream that requires treatment and contributes to volatile organic emissions to the air. The present invention relates to a less costly polymerization process that can be carried out at atmospheric pressure.
Atmospheric pressure processes employing an inert gas have been disclosed in the prior art, but these suffer from one or more drawbacks such as (1) the quantity of inert gas used is too large to be economical; (2) the reactor size might not be feasible for commercial-scale operation; (3) inert-gas velocities may be too high to be feasible for commercial-scale production, or (4) contact between the inert gas and the polymer melt in the reactor be inadequate or non-uniform. Because of such drawbacks, the processes presently employed for commercial production of polyester continue to be conducted under high vacuum. One object of the present invention is to provide further improvement in a process, at about atmospheric pressure, for continuous or batchwise production of polyesters, particularly polyethylene terephthalate, of high molecular weight. In another aspect of the present invention, an improved apparatus that may be employed in a reaction process involving mass transfer of a volatile by-product into an inert gas, is disclosed.